This invention relates to an engine control apparatus, or more in particular, to a control apparatus for reducing the emission gas at the time of starting the engine.
With the recent strengthened control of the emission gas of the automotive vehicle engine in North America, Europe and Japan, a further reduction in engine emission gas has been required. The catalyst performance has been improved and the accuracy of catalyst control has been increased to such an extent that the greater amount of the emission gas from the engine is discharged at the time of starting. The emission performance depends to a large measure on the air-fuel ratio in the cylinder, and therefore, the optimization of the air-fuel ratio in the cylinder at the time of starting the engine is the most important subject.
JP-A-2-227526 discloses an invention wherein in order to properly control the air-fuel ratio of the engine started at a very low temperature, the fuel injection amount (target air-fuel ratio) is set to the richer side than normal, and after starting, shifted gradually to the lean side and returned to the normal air-fuel ratio.
In the engine for injecting the fuel through the intake port, the amount of the fuel injected and flowing into the cylinder is known to change in accordance with the engine temperature and the intake air temperature at the time of starting. Especially, with the decrease in temperature, the amount of the fuel flowing into the cylinder decreases, and therefore, the air-fuel ratio in the cylinder becomes more difficult to optimize.
With the decrease in temperature, the amount of the injected fuel which flows into the cylinder decreases. Especially at a very low temperature below the freezing point, the flow rate into the cylinder is extremely reduced. The amount of air flowing into the cylinder, on the other hand, undergoes substantially no change. Rather, the air density increases with the decrease in temperature, often resulting in a higher efficiency of filling up the air in the cylinder. According to the invention cited above, therefore, a greater amount of fuel is required to be injected to optimize the air-fuel ratio at the time of starting the engine at a low temperature. The injection of a greater amount of fuel, however, leaves a great amount of fuel in the intake pipe, and the fuel remaining in the intake pipe gradually flows into the cylinder after starting, thereby deteriorating the air-fuel ratio controllability. The amount of the fuel that can be injected by the injector (fuel injection valve) per cycle is limited by the dynamic range of the injector and the engine stroke (since the engine is rotated by cranking, the period during which the fuel can be injected is limited). Especially at a very low temperature, therefore, the fuel cannot be supplied in an amount commensurate with the amount of the air flowing into the cylinder.
For the reason described above, the optimization of the air-fuel ratio at the time of starting the fuel injection as intended by the cited invention is limited at a very low temperature.
JP-A-10-54271, on the other hand, discloses the invention of a fuel injection control apparatus for injecting the fuel in accordance with the timing of the intake stroke, wherein the fuel injection amount (target air-fuel ratio) for the first fuel injection session is set separately from the fuel injection amount for the second and subsequent fuel injection sessions.
In the engine for injecting the fuel into the intake port, the amount of the fuel injected into the intake pipe which flows into the cylinder is known to change in accordance with the engine temperature at the time of starting, the intake air temperature or the fuel property. In order to optimize the air-fuel ratio in the cylinder, the fuel is required to be controlled highly accurately taking the fuel behavior at the time of starting into consideration.
In the invention cited above, the effect of the attached fuel is taken into consideration only for the first injection session. The change in the amount of the attached fuel which may occur after the first injection session, therefore, is not taken into consideration, with the result that the amount of the fuel flowing into the cylinder develops an error. Once the amount of the fuel flowing into the cylinder develops an error, the air-fuel ratio for combustion becomes erroneous and the emission is deteriorated. The mechanism and the factors for determining the amount of the attached fuel are yet to be made apparent. In spite of the known fact that the amount of the attached fuel has some effect, therefore, the accuracy of the fuel control operation taking the amount of the attached fuel into consideration is still not considered sufficiently high.
Also, the cited invention presupposes the fuel injection in accordance with the intake stroke. The fuel injected at the intake stroke, however, is liable to flow directly into the cylinder without being temporarily formed into a liquid film on the surface of the inner wall of the intake pipe or the surface of the intake valve. In such a case, the shearing stress of the intake air velocity cannot be fully utilized, and therefore, the fuel enters the cylinder without being sufficiently pulverized. The resulting larger particle size of the fuel deteriorates the combustion characteristic, thereby adversely affecting both the torque performance and the emission performance.